There seems to be multiple points of view on this matter. One may say that his insertion of Coruscant and other EU elements in the prequels are an indication that he is involved heavily. Not to mention his say-so's on certain story elements in EU prose (Anakin dying instead of Jacen, no more post ROTJ Wookie Jedi, Yuuzhan Vong being original villains instead of Proto-Sith, etc...) But then we've got stuff like the TCW cartoon, and what may possibly come in Episode VII. Does that mean he doesn't give two siths about the EU? Continuity is a lie, there is only Word of Lucas Through Word of Lucas, I gain Ownership Through Ownership, I gain Power Through Power, I gain Sole Creative License Through Sole Creative License, my EU chains are broken The Sequels will set me free.
Are you asking what influence Lucas had on EU before Disney bought Star Wars? Because if not, see Immodet's comment.
Before, we tend to hear about what George does or doesn't do. We know he loved Dark Empire, know he vetoed the Noghri or Yuuzhan Vong being the Sith (species), know he vetoed Luke dying in Vector Prime, and ordered Anakin Solo's death in the NJO to avoid the confusion between Anakin Skywalker and Anakin Solo... and we know he pushed the creators of Force Unleashed away from Plagueis and towards Leia and Palpatine... oh, and he personally wrote the Mortis Trilogy. He also authorised Maul's return, Piell's death and of course, Adi's demise... killing some parts of the Expanded Universe canon. So, it varied. He nodded through the majority of it all, but had shown a lot of interest in TCW.
